The field of the invention is multi-component thin sheath fibers.
Multi-component fibers have found numerous applications in various products, including carpet fibers, fibers that are exposed to mechanical stress and fibers that are exposed to environmental stress, and among such fibers, sheath core fibers (i.e., fibers with a core that is surrounded by a sheath) can in many cases be manufactured in relatively large scale. However, production of such fibers becomes increasingly difficult as the thickness of the sheath decreases.
For example, decreasing sheath thickness frequently leads to inhomogeneity of the overall sheath thickness in various sheath core fibers. One approach to reduce inhomogeneity of a sheath is described in EP 0 011 954 B1 to Perkin, disclosing a configuration and spinning conditions that increase the degree of homogeneity of sheath thickness within and among a population of fibers. Although Perkin""s spinning apparatus improves the degree of homogeneity (e.g., approximately 15% of the fibers have the desired sheath content of 15% while the sheath content of the remaining fibers varies between 5% and 15% and 15% and 30%), sheath homogeneity still remains problematic.
In another approach, Lijten et al. employ a process in which at least in the area surrounding the stream of a core component the sheath component is subjected to a flow resistance as described in U.S. Pat. No. 5,618,479. Although Lijten""s process significantly improves homogeneity of sheath thickness as compared to Perkin""s fibers, homogeneity of sheath thickness still remains problematic, especially where the sheath thickness is less than 10% (e.g., 60% of fibers have a sheath content of 9%xc2x11%).
A further problem of known spinning processes for production of sheath core fibers is that such processes typically limit the choice of materials to polymers with substantially similar rheological properties. Consequently, many sheath core fibers employ the same or almost the same polymeric material, which may then be modified with an additive to impart a particularly desirable characteristic into the fiber (see e.g., U.S. Pat. No. 6,174,603 to Berger, or U.S. Pat. No. 5,827,611 to Forbes). Among various other characteristics, resistance to solvents and other relatively aggressive chemical agents is often particularly desirable. In one approach, a particularly desirable characteristic may be imparted into the fiber by incorporating relatively large quantities of an additive into the fiber. However, relatively high concentrations often reduce tenacity and/or other mechanical properties.
Alternatively, the fiber may be surface-coated with the additive to achieve a particularly high concentration of the additive on the fiber. While coating typically allows introducing substantial amounts of the additive onto the fiber, coatings are generally prone to abrasion. To overcome at least some of the problems associated with abrasion, the surface of a fiber may be chemically derivatized to couple the additive to the fiber surface. Although chemical surface modification often improves abrasion resistance, chemical surface modification may alter one or more physico-chemical surface properties, thereby potentially interfering with subsequent production steps.
Although various sheath-core fibers are known in the art, all or almost all of them suffer from one or more problems, especially as the thickness of the sheath decreases. Thus, there is still a need to provide improved sheath core fibers.
The present invention is directed to methods and compositions for sheath core fibers with a core formed from a core material and a sheath formed from a sheath material comprising a fluoropolymer, wherein the sheath at least partially surrounds the core.
In one aspect of the inventive subject matter, the sheath material has a apparent shear viscosity VS that is equal to or less than the apparent shear viscosity of the core material VC, wherein VC is at least 1.3 times VS, and more preferably at least 1.6 times VS. Further preferred sheath materials include melt-processable fluoropolymers, and especially contemplated fluoropolymers are poly(vinylidene fluoride) (PVDF), ethylene-chloro-tri-fluoro-ethylene (ECTFE), and ethylene-tetrafluoro-ethylene (ETFE).
In another aspect of the inventive subject matter, contemplated core materials comprise an organic polymer, preferably poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), a polyamide, or a polyolefin. The core of particularly preferred sheath core fibers is at least 70 wt %, more preferably at least 80 wt %, and most preferably at least 90 wt % of the fiber.
In a still further aspect of the inventive subject matter, a method of producing a fiber has one step in which a core material, and a sheath material that comprises a melt-processable fluorine-containing polymer are provided. In a still further step, a spin pack is provided and a sheath core fiber with a sheath and a core is formed from the sheath material and the core material using the spin pack, wherein the sheath at least partially surrounds the core. In especially contemplated fibers and methods, the core has a weight WC, the sheath has a weight WS, WS/WC is no higher than 0.43, more preferably no higher than 0.25, and most preferably no higher than 0.12, and the spin pack has a sheath material conduit having a ratio of open volume to sheath material mass flow of equal or less than 1.13, 1.7, or 3.4, respectively.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.